Wireless receivers are in popular use for data communications in, for example, the 2.4 GHz and 5 GHz bands in compliance with the IEEE 802.11a, IEEE 802.11b, and IEEE 802.11g standards. Mobile phones provide good examples. The wireless receiver amplifies an incoming wireless signal with an amplifier circuit tuned to a predetermined wireless frequency. The amplifier circuit in the wireless receiver is typically capable of receiving weak signals of about −90 dBm.
The incoming signal has a strength which is variable with the distance between the receiver and the transmitter where the signal originates. For example, if the transmitter is located close to the receiver, the signal level as received on the antenna is about −5 dBm. The necessary input signal can become weaker than unwanted signals in nearby channels.
The amplifier circuit is required to provide a flat signal characteristics profile and high linearity over a wide range of input signal frequencies. Specifically, the amplifier circuit desirably amplifies low level input signals (−90 dBm to −60 dBm) at high gain with low noise and high level input signals (−60 dBm or higher) at high gain and high linearity.
Generally, these requirements are met by a variable gain amplifier circuit which allows for a variable gain setting. Receivers incorporating a variable gain amplifier circuit detect input signal strength and automatically change the gain of the variable gain amplifier circuit.
Conventional variable gain amplifier circuit come in various forms which will be now described in reference to FIG. 11 to FIG. 17.
U.S. Pat. No. 6,600,371 (published Jul. 29, 2003) discloses the conventional circuit shown in FIG. 11. The illustrated circuit is a quad amplifier containing four transistors Q112, Q113, Q112′, and Q113′. The four transistors amplify signals from major signal paths while controlling gains. A gain setting is made through control voltage.
FIG. 12 shows an exemplary conventional circuit. The illustrated conventional circuit is a wireless signal transmitter/receiver. The circuit provides a Gilbert cascode stage containing an input transistor Q121 with an emitter feedback as well as transistors Q122 and Q123. The Gilbert cascode stage processes signals either to or from the output section Vout. This means that the Gilbert cascode stage controls the amount of signal traversing from input Vin to output Vout, by adjusting the controlling voltages Vc1 and Vc2.
U.S. Pat. No. 6,566,963 (published May 20, 2003) discloses the circuit shown in FIG. 13. The conventional circuit has two amplifier stages. Specifically, in the conventional circuit, the first amplifier stage involving a transistor Q131 connects to the second amplifier stage (transistors Q132, Q133, and Q134) via a transformer 131.
FIG. 14 shows another conventional circuit example. The illustrated conventional circuit has two signal paths. A first signal path consists of transistors Q141, Q142, and Q143 an has high gain. A second signal path consists of Q144 and has low gain and high linearity in signal conversion. Along the first signal path, the gain can be varied by the transistors Q142, Q143. As to the second signal path, the transistor Q144 is a common base configuration. The resistor R141 helps to improve the linearity of the transistor Q144.
The conventional circuit shown in FIG. 15 is disclosed in “A 2.5-GHz BiCMOS transceiver for wireless LAN's” by R. G. Meyer et al., IEEE Journal of Solid-State Circuits, Vol. 32, No. 12, pp. 2097–2104, December 1997. Like the one in FIG. 14, this conventional circuit has two signal paths. A first signal path consists of a transistor Q154 and has high gain. A second signal path consists of transistors Q151, M152 and has low gain. Both transistors Q151, M152 act as an emitter follower. When the first signal path is used to amplify an input signal, the transistor Q154 acts on the signal. On the other hand, when the second signal path is used to amplify an input signal, the transistors Q151, M152 act on the signal; the transistor Q154 is turned off.
The conventional circuit shown in FIG. 16 is disclosed in “A direct-conversion receiver for 900 MHz (ISM Band) spread-spectrum digital cordless telephone” by C. D. Hull et al., IEEE Journal of Solid-State Circuits, Vol. 31, No. 1a2, pp. 1955–1963, December 1996. Like the one in FIG. 12, the illustrated conventional circuit relies on transistors Q164, Q167 for variable gain. The degree of linearity of the circuit is determined by the characteristics of the transistor Q164.
The conventional circuit shown in FIG. 17 is disclosed in Published Japanese translation of PCT application 2002-506304 (Tokuhyo 2002-506304; published Feb. 26, 2002). The illustrated circuit can vary its gain because of a change of bias current Ibias to digital.
These conventional circuits however have a narrow dynamic range, variable input impedance dependent on the gain setting, and other problems.
The conventional circuit in FIG. 11 has another problem that a balanced unbalanced device (balun) is needed to couple the input signal from an antenna to the amplifier stage. Another problem is that resistor and transistor parameters among others must be altered to suit the operating wireless frequency.
The conventional circuits shown in FIGS. 12, 13, 16 shows a narrow dynamic range when the input signal has large amplitude, which can be a problem.
The FIG. 14 conventional circuit has other problems: The circuit does not show a constant input impedance when the signal paths are switched. The resistor R141 adds to the noise factor. The input signal inverts in polarity depending on the operating signal path. Concretely, in the FIG. 14 circuit, the transistor Q141 is an inverting amplifier. The input signal inverts its polarity when it passes through the transistors Q141, Q142. In contrast, the transistor Q144 is a common base configuration; the input signal does not invert its polarity when it goes through the path involving the transistor Q144.
As for the FIG. 15 conventional circuit, the input impedance does not match the signal source impedance. An external matching network is needed for the impedance matching, which adds to the component count. Another problem is found where the input signal has different polarities between the two signal paths, because the transistor Q154 is an inverting amplifier, and the transistor M152 is a non-inverting amplifier. Also, the parasitic capacitance due to the presence of the transistors M151, M152 can degrade noise factor.
In the FIG. 17 conventional circuit, the input impedance, which is a function of the bias current Ibias, is dependent on the gain setting and varies greatly. Therefore, the input impedance does not match the signal source impedance.